One of the main arguments made by vegans and vegetarians supporting their food choice relates to not causing animals pain. The argument is quite compelling, especially given many modern, high volume, farming methods - but it is usually hypocritical, in that the defence of animals rarely extends to insects and other less "cute" animals.
Some recent studies now seem to close down the argument on plants not feeling pain. The Economist has a very interesting article reviewing work showing, experimentally, that not only do plants feel pain, but they also have the capacity to remember the circumstances when they felt pain. Another article in Wired, had a discussion on plants communicating warning between each other - although this was restricted in terms of location and species of plants.
This is quite interesting beyond just the concept of plants being sentient (although I think Ents are a bit far off). It extends our own understanding of what biological constructs may exist for nervous systems - and that the traditional known nervous systems are not the only ones. Perhaps, in one of the planets out there, there is a sentient species that are just like Ents :)